Callus or hard skin is a thickening of keratinised layers of the skin. The most common cause of this is mechanical stress, pressure, friction, shearing etc.
It is a normal physiological process and a protective response.
It becomes pathological when it is painful, and then needs treatment. This consists of debridement by scalpel.
Corns (helomata) are small areas of callus that contain a nucleus which press down and causes pain; they are caused by concentrated intermittent pressure.
They are treated by the removal of overlaying callus and enucleation of the nucleus by scalpel.
Fissures in callus are splits in the epidermis that are due to loss of elasticity, primarily because the skin is thick and dry.
Treatment is the removal of the callus and the rehydration of the skin with an emollient that has a high urea, lactic acid content (25%).
At home it is possible to reduce callus with a foot file, this should be done when the skin is dry prior to bathing, and an emollient such as aqueous cream applied after bathing (not between the toes).